One of the common constituents in ceramic articles made from clays is mullite (Al.sub.6 Si.sub.2 O.sub.13). This is produced in the firing process by the breakdown of the clay and growth of needle-like mullite crystals. Mullite has many desirable properties. These include a low coefficient of thermal expansion, high strength, high electronic resistivity, hardness and insolubility in strong acids and bases. In porcelain and china, the development of mullite is responsible for its high strength and hardness. In refractories, mullite contributes thermal shock resistance, high strength, and hot strength. Mullite is a strong, versatile refractory. Its high dielectric breakdown resistance make it an excellent insulator.
Attempts to manufacture mullite from natural materials such as clays and bauxites result in undesirable impurities which come from the natural raw materials. Attempts to manufacture high purity mullite from aluminum oxide and silica are very expensive because the alumina and silica must diffuse together through the mullite which forms at the interface between the two. This diffusion is so slow that repeated heat treatment with grinding between each heat treatment is required. The temperatures must be at least 1600.degree. C. and usually are higher. There is therefore a recognized need for a practical commercial process for preparing high purity mullite. This has not heretofore been met from the general state of knowledge of sintering oxides. (See, for example, Greskovich and Woods, "Fabrication of ThO.sub.2 -Doped Y.sub.2 O.sub.3 ", Ceramic Bulletin, Vol. 52, No. 5, pp. 473-378 (1973).
It is known that salts of aluminum and silicon can be mixed in a non-aqueous solution to form a coprecipitate of aluminum and silicon hydroxides on the addition of aqueous ammonium hydroxide. When the aluminum and silicon are present in the chemical ratio to form mullite, the precipitated hydroxide gel can be dehydrated by drying to produce a powder containing molecularly intermixed alumina and silica, which on firing at a high temperature can be converted to mullite. M. O. Marlowe and T. D. McGee, "Analysis of Fe.sub.2 O.sub.3, TiO.sub.2, and Cr.sub.2 O.sub.3 in Mullite by O-Ray Fluorescense," Proc. Iowa Acad. Sci., 70, 153-60 (1963). The resulting mullite, however, is not suitable for use as a ceramic product, since the mullite is in the form of large, coarse crystals, whereas a dense, integrated body of fine mullite crystals is required for commercially acceptable mullite ceramics. Further studies on "Mullitization of Alumino-Silicate Gels" were reported by T. D. McGee and C. D. Wirkus, in The American Ceramics Society Bulletin, Vol. 51, No. 7, 577-581 (July, 1972). The experiments of that paper had been previously presented by me at the 23rd Pacific Coast Regional Meeting of the American Ceramics Society, San Francisco, California, Oct. 28, 1970. In carrying forward the investigation of my 1963 paper (above cited), it was shown that the molecular scale mixing in non-aqueous solution prior to coprecipitation permits complete conversion of alumino-silicates to high purity mullite at temperatures of around 1150.degree. C. In some of the reported experiments, titanium hydroxide was incorporated in the precipitated gel in amounts corresponding to about 3 to 12% titania by weight based on the resulting mullite-titania ceramic, the TiO.sub.2 replacing a corresponding molar amount of SiO.sub.2. Crystalline TiO.sub.2 was shown to be present at 3% TiO.sub.2 or greater, but the increase in cell size at 3% TiO.sub.2 as compared with 0% indicated that some of the mullite was in solid solution. The conclusion was therefore reached that "titania has little solid solubility in mullite at 1300.degree. C. and inhibits crystal growth."
The extent of solid solution solubility of titania in mullite had previously been investigated. See G. Gelsdorf and H. Schwiete, Arch. Eisenhuttenwes., 27, 807-811 (1956). Gelsdorf and Schwiete had determined by X-Ray defraction investigation that "1.5% TiO.sub.2 at 1450.degree. C. can be incorporated in the mullite lattice," referring to the mechanism of incorporation as "Einlagerung" (intercalation). However, the present invention is based on the discovery that the solid solution solubility of titania in mullite is much lower than 1.5%, and that concentrations of as little as 0.5 to 1% titania can act as an effective grain growth inhibitor for mullite crystals. The invention further includes the development of a specific set of processing steps, involving critical calcining temperatures, whereby integrated, high density mullite bodies can be produced. Final densities in the range of 2.50 to 3.16 grams per cubic centimeter are obtainable. If higher concentrations of titania had been employed, as taught by the prior art, the resulting density of the final product under comparable processing conditions would be higher. Therefore, where the objective is to produce mullite bodies of maximum density, as with the process of the present invention, the use of a critically small portion of titania is therefore essential, but prior to the present invention this was not known to be possible, and, in fact, the state of the art suggested that amounts of titania of 1.5% or less based on the mullite would be ineffective as an inhibitor of mullite grain growth.